Silicone-based cyanate-ester cross-linkable die attach adhesive

ABSTRACT

The present invention describes a method including: providing a material A, the material A including a siloxane backbone with a hydride functional group; reacting the material A with a material B in the presence of a catalyst to form a material C, the material B including an alkenyl functional group and an aromatic carbonate functional group; heating the material C to form a material D, the material D including a phenol functional group; and reacting the material D with a material E and a material F to form a material G, the material E including a cyanogen halide, the material F including an acid acceptor, the material G including an aromatic cyanate ester functional group. 
     The present invention further describes a die attach adhesive including a three-dimensional network of substituted triazine rings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit manufacturing, and more specifically, to a die attach adhesiveand a method of forming such an adhesive.

2. Discussion of Related Art

As the density for non-CPU chips continues to increase, the thickness ofthe package for the chips must be reduced to accommodate the stacking ofmultiple die on a single substrate form. Consequently, the bondlinethickness (BLT), or thickness of the adhesive between the chip and thesubstrate, must also be decreased. However, the die attach material mustcontinue to serve as a buffer layer between the chip and the substrateto relieve any stress from mismatch in the coefficient of thermalexpansion (CTE) which would otherwise cause warpage, cracking, ordelamination of the chip.

The latest trends in packaging also involve the elimination of Lead fromsolder and the raising of the reflow temperature to 260 degreesCentigrade. As a result, the die attach material must be stable at thehigher temperatures and be resistant to high humidity while maintaininggood adhesion between the chip and the substrate.

Thus, what is needed is a die attach adhesive and a method of formingsuch an adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a polysiloxane with hydride functionalgroup according to the present invention.

FIG. 2 is an illustration of a styrene substituted with tertiary-butylcarbonate end group according to the present invention.

FIG. 3 is an illustration of a polyorganosiloxane with tertiary-butylcarbonate end group according to the present invention.

FIG. 4 is an illustration of a polyorganosiloxane with phenol end groupaccording to the present invention.

FIG. 5 is an illustration of isobutylene as gaseous byproduct accordingto the present invention.

FIG. 6 is an illustration of triethylamine as acid acceptor according tothe present invention.

FIG. 7 is an illustration of a polyorganosiloxane with aromatic cyanateester end group according to the present invention.

FIG. 8 is an illustration of substituted triazine rings according to thepresent invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description, numerous details, such as specificmaterials, dimensions, and processes, are set forth in order to providea thorough understanding of the present invention. However, one skilledin the art will realize that the invention may be practiced withoutthese particular details. In other instances, well-known semiconductorequipment and processes have not been described in particular detail soas to avoid obscuring the present invention.

The present invention describes various embodiments of a method offorming a cyanate-ester-based silicone adhesive, a composition ofcyanate-ester-based silicone adhesive, a method of die attach withcyanate-ester-based silicone adhesive, and a die-substrate package withsubstituted-triazine-ring silicone adhesive having reduced bond linethickness (BLT).

A polymer is a macromolecule that is formed from many small and simplerepeating units, or mers. A polymer may be monodisperse or polydisperse.Dispersion refers to the distribution of molecular weights (MW) which isrelated to the degree of polymerization (DP). A compound that includesSi—O—Si linkage is called a siloxane. A polymer of siloxane is called apolysiloxane.

A polymer may include one or more chains. The principal or main chain,usually the longest, of the polymer is called the backbone. Othersecondary chains, usually shorter, that may be attached to the backboneby a covalent bond may be considered as branches of the principal chain.A covalent bond is a directional bond joining atoms. A polymer with onlyone chain has a linear backbone while a polymer with more than one chainhas a branched backbone. A linear backbone may have a zig-zag shape dueto the bond angles.

A chain may include one or more types of functional groups. Likebranches, functional groups are attached to a chain by a covalent bond,but, unlike branches, functional groups may be short and may repeatalong the chain. A functional group that is attached to a chain in aterminal position is considered an end group while a functional groupthat is attached to a chain in a non-terminal position is considered apendant group.

The functional group may be small or large. The functional group may becompact or bulky. The functional group may include a lyophilic alkylgroup. An organic-substituted polysiloxane is called apolyorganosiloxane (POS). Polyorganosiloxanes have also been widelyreferred to as silicones ever since F. S. Kipping incorrectly chose thisname in the 1920s.

Material A is a polyorganosiloxane, or silicone, with hydride functionalgroups. In an embodiment of the present invention, material A, as shownin FIG. 1, includes a linear siloxane backbone with hydride functionalgroups. In another embodiment of the present invention, material A is adimethylsiloxane (DMS) homopolymer with hydride end groups, in whichboth R₁ and R₂ pendant groups are methyl groups, CH₃. A homopolymer is apolymer that includes only one type of repeating unit, or mer.

A first example of a DMS homopolymer is DMS-H03 with an averagemolecular weight of about 400-500 and a viscosity of about 2-3Centipoise (Cps). A second example is DMS-H25 with an average molecularweight of about 17,200 and a viscosity of about 500 Cps. A third exampleis DMS-H41 with an average molecular weight of about 62,700 and aviscosity of about 10,000 Cps.

Viscosity is a measure of the resistance to flow of a polymer in asolution or in a melt. Viscosity of the polymer varies as a function ofsize and shape (conformation) of the polymer. Viscosity of the polymeralso varies as a function of temperature.

In still another embodiment of the present invention, material Aincludes a branched siloxane backbone with hydride functional groups,such as a methylhydride-dimethylsiloxane (MH-DMS) alternating copolymerwith hydride end groups, in which R₁ pendant group is a methyl group,CH₃, while R₁ pendant group alternates between a methyl group, CH₃, anda hydride group, H.

A copolymer is a polymer that includes two or more types of repeatingunits, or mers. A copolymer of polymer M₁ and polymer M₂ may include analternating copolymer, (M₁M₂)_(n), a random (or statistical) copolymer,a block copolymer, (M₁)_(n)(M₂)_(n), and a graft copolymer, such as(M₁M₁(M₂)_(n)M₁)_(n).

A first example of an MH-DMS alternating copolymer is HMS-013 with anaverage molecular weight of about 55,000 and a viscosity of about6,000-8,000 Cps. A second example is HMS-071 with an average molecularweight of about 1,900-2,000 and a viscosity of about 25-35 Cps. A thirdexample is HMS-501 with an average molecular weight of about 900-1,200and a viscosity of about 10-15 Cps.

In a further embodiment of the present invention, material A includes acyclic siloxane backbone with hydride functional groups. An example is asilsesquioxane polymer which may be a three-dimensional molecule with aregular or irregular cage-like structure.

Material B may be an aliphatic or an aromatic compound that includes analkenyl end group and a carbonate end group. An aliphatic compoundrefers to a linear organic compound. An aromatic compound refers to acyclic organic compound. The alkenyl end group may include a vinyl,allyl, butenyl, pentenyl, or an hexenyl end group.

In an embodiment of the present invention, material B includes analkenyl end group and an alkyl carbonate end group. In anotherembodiment of the present invention, material B includes an alkenyl endgroup and an aromatic carbonate end group. In still another embodimentof the present invention, as shown in FIG. 2, material B may be astyrene substituted with a tertiary-butyl carbonate end group,(CH₃)₃COCOO(C₆H₄)CHCH₂.

In an embodiment of the present invention, a hydride functional group,that is terminal and/or pendant, from material A is added to an alkenyl,such as a vinyl, end group of material B in the presence of a catalyst.A catalyst is a substance that alters the rate of a chemical reactionwithout itself undergoing a permanent change in composition.

In an embodiment of the present invention, about 100 parts per million(ppm) or less of the catalyst is used. In an embodiment of the presentinvention, the catalyst may include a metal, such as Platinum orRhodium. In another embodiment of the present invention, the catalystmay include a metal complex, such as Platinum carbonylcyclovinylmethylsiloxane complex, Platinum divinyltetramethyldisiloxanecomplex, Platinum octanaldehyde/octanol complex, andtris(dibutylsulfide) Rhodium complex.

The reaction between material A and material B will produce material C.In an embodiment of the present invention, material C is apolyorganosiloxane, or silicone, having a linear or branched or cyclicbackbone, with tertiary-butyl carbonate functional groups that may beterminal, as shown in an embodiment of the present invention in FIG. 3,and/or pendant (not shown). In another embodiment of the presentinvention, material C may be polyfunctional with two or moretertiary-butyl carbonate functional groups per molecule.

Material C is then heated to a temperature, such as about 100-150degrees Centigrade, to release tertiary-butyl carbonate functionalgroups and form material D. The temperature may be higher when materialB is an aliphatic compound and lower when material B is an aromaticcompound. In an embodiment of the present invention, material D is apolyorganosiloxane, or silicone, having a linear or branched or cyclicbackbone, with phenol functional groups that may be terminal, as shownin FIG. 4, and/or pendant (not shown). In another embodiment of thepresent invention, material D may be polyfunctional with two or morephenol functional groups per molecule.

Heating also breaks down the tertiary-butyl carbonate functional groups,whether terminal and/or pendant, to form byproducts, such asisobutylene, (CH₃)₂CCH₂, as shown in FIG. 5, and carbon dioxide, CO₂.The isobutylene and carbon dioxide may be evolved as gaseous byproducts.

Material D may also be formed in other ways. In an embodiment of thepresent invention, material D may be formed by reacting silazane withprotected alcohol functional groups that are terminal and/or pendant andthen cleaving the protecting groups to produce phenol functional groupsthat are terminal and/or pendant.

Material D is then reacted with material E and material F to formmaterial G. In an embodiment of the present invention, a lowtemperature, such as about 0 degree Centigrade, is used since,otherwise, reactivity may be too high.

Material E may be a cyanogen halide. In a first embodiment of thepresent invention, cyanogen bromide (BrCN) may be used. In anotherembodiment of the present invention, cyanogen chloride (ClCN) may beused. In still another embodiment of the present invention, cyanogeniodide (ICN) may be used.

Material F is an acid acceptor, such as a base. In an embodiment of thepresent invention, material F may include a linear or branched alkylamine, such as triethylamine, (CH₃CH₂)₃N. In other embodiments of thepresent invention, the alkyl group may include methyl, n-propyl,isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, octyl, undecyl, andoctadecyl.

The reaction of material D and material E will produce material G and anacidic byproduct. The acidic byproduct will react with material F toform a halide salt. The halide salt may be removed by washing withwater. Ions and halogens should be removed to prevent corrosion.

In an embodiment of the present invention, material G is apolyorganosiloxane, or silicone, having a linear or branched or cyclicbackbone, with aromatic cyanate ester functional groups, that may beterminal, as shown in FIG. 7, and/or pendant. In another embodiment ofthe present invention, material G may be polyfunctional with two or morearomatic cyanate ester functional groups per molecule.

Material G may enhance performance and reliability of an electronicpackage. In an embodiment of the present invention, material G may serveas an underfill, such as for a flip chip. In another embodiment of thepresent invention, material G may serve as an encapsulant. In stillanother embodiment of the present invention, material G may serve as anadhesive, such as for die attach.

An adhesive may attach a die to a surface of a substrate. The surface ofthe substrate may or may not be metallized with a wiring pattern. In anembodiment of the present invention, the surface of the substrate is thefloor of a cavity that is defined by one or more tiers surrounding thesurface. In another embodiment of the present invention, the substratehas no cavity and the surface is an entire side of the substrate.

The die attach adhesive provides a mechanical and thermal interfacebetween the die and the substrate. The die may be formed from asemiconductor material, such as silicon. In an embodiment of the presentinvention, the substrate is formed from a ceramic material to provide astable dielectric constant, a low dielectric loss tangent, and goodthermal dissipation. In another embodiment of the present invention, thesubstrate is formed from an organic material to provide a low dielectricconstant, a lighter weight, and a low cost.

Material G may be a solvent-free paste so solvent removal is notrequired during curing. In an embodiment of the present invention,material G has the appropriate viscosity and surface tension to wet thesurface of the substrate. In another embodiment of the presentinvention, material G has the appropriate viscosity and surface tensionto wet the backside of the die. Good adhesion requires penetration ofmaterial G into crevices or pores at the surface of the substrate and/orat the backside of the die. As needed, contamination at the surface ofthe substrate and at the backside of the die may be removed, such as byArgon plasma clean, before dispensing the die attach adhesive.

A dispense pattern for the die attach adhesive may depend on the sizeand shape of the die and the size and shape of the substrate, such asthe cavity. The dispense pattern must ensure a bond line that is free ofvoids. In an embodiment of the present invention, dispensing a series ofdots may produce a flatter and more evenly contoured dispense thandispensing a continuous line. In another embodiment of the presentinvention, the outermost dots may be larger than the inner dots so as toprovide a flatter dispense.

The dispense pressure and the syringe tip speed should be controlled todeliver the proper volume of the die attach adhesive. In an embodimentof the present invention, the needle of the syringe is programmed tomove back over the dispensed portion of the die attach adhesive beforemoving up to its highest set position so as to avoid tailing.

Application of the die attach adhesive is followed by die placement. Dieplacement onto the dispensed die attach adhesive must be performedprecisely with respect to placement accuracy, orientation, andplanarity. Applying pressure to set the die onto the adhesive may helpto establish good adhesion and proper bond line thickness.

Die placement is followed by curing of the die attach adhesive at anappropriate temperature for an appropriate duration of time to bond thesurface of the substrate to the backside of the die. Material G maypossess two or more aromatic cyanate ester functional groups that may beterminal and/or pendant per molecule.

Material G may be heated to a temperature, such as about 150 degreesCentigrade or higher, in the presence of a curing catalyst to cross-linkand form material K. Curing is a process, induced by exposure to heat orlight, that involves a chemical reaction and requires a catalyst.Cross-linking involves formation of covalent bonds that may beintermolecular or intramolecular.

Material G undergoes addition polymerization during the cure to formmaterial K. No leaving groups are produced. Volatile byproducts areundesirable since they may form voids that result in a porous and weakbond line interface.

In an embodiment of the present invention, the curing catalyst mayinclude an organometallic compound. Organometallic refers to an organicgroup that includes at least one carbon atom which is bonded to atransition metal atom. A transition metal is an element that includes1-10 electrons in a “d” sublevel and is classified in a group (a columndesignated “B” in the United States) near the center of the PeriodicTable of the Elements.

In another embodiment of the present invention, the curing catalyst mayinclude a metal chelate and/or a metal salt. A chelate is a heterocycliccompound having a central metallic ion covalently bonded to two or morenonmetallic atoms in the same molecule. The metal may be divalent (II),trivalent (III), or tetravalent (IV). Examples of metals include Copper,Zinc, Cobalt, Manganese, Tin, Nickel, Titanium, Aluminum, and Lead.Examples of ligands include napthenates, octoates, octylates, andacetylacetonates. A ligand is a molecule, ion, or atom, that iscovalently bonded to the central metal compound of a coordinationcompound.

The curing catalyst may be provided in a solution of material H.Material H includes an alkyl liquid phenol, such as nonylphenol. In anembodiment of the present invention, the metal concentration may beabout 0.0025-0.0200 parts by weight on the basis of 100 parts by weightof polymer (resin), depending on pot life and curing requirements. In anembodiment of the present invention, about 5 percent by weight or lessof the curing catalyst is used. In another embodiment of the presentinvention, about 1-2 percent by weight of the curing catalyst may beused.

In an embodiment of the present invention, curing of material G willresult in cyclotrimerization of material G to form material K with athree-dimensional network of substituted triazine rings. Each triazinering, (ArOCN)₃, is formed from an end group or pendant group of 1-3molecules of material G. The triazine ring is a heterocyclic ring. Aheterocyclic ring is a ring compound that includes carbon, as well asnon-carbon atoms, such as nitrogen (in this case), oxygen, and sulfur.

Material K, as shown in FIG. 8, with the three-dimensional network,bonds the surface of the substrate to the backside of the die. Adhesivestrength may be determined by measuring peeling strength at a boundarybetween the die and the substrate. Material K may have an adhesivestrength of about 1-10 MPa.

In an embodiment of the present invention, other (non-G) materials maybe present when material G is cured to form material K. The non-Gmaterials may include cross-linking agents, extending agents, andhardening agents. The non-G materials may be monofunctional,bifunctional, or polyfunctional. Monofunctional monomers react to formlow-molecular-weight products. Bifunctional monomers react to formlinear polymers. Polyfunctional monomers react to form branched orcross-linked (three-dimensional) polymers.

The non-G materials may include cyanate ester functional groups or othertypes of functional groups, such as hydroxyl (OH), amine (NH), cyanate(CNO), and epoxy. In an embodiment of the present invention, thefunctional groups on the non-G materials may react and co-cure with thearomatic cyanate ester pendant and/or end groups of material G.

In another embodiment of the present invention, functional groups on thedie or substrate (mold compound) may react and co-cure with the aromaticcyanate ester pendant and/or end group of material G to improve adhesionto the die or substrate. In an embodiment of the present invention, thearomatic cyanate ester pendant and/or end group of material G maycondense with a hydroxyl (OH) functional group on the die or thesubstrate (mold compound) to form an imidocarbonate linkage. In anembodiment of the present invention, the imidocarbonate linkage may bethermally reversible.

In yet another embodiment of the present invention, ambient moisture orwater, H₂O, may react with the aromatic cyanate ester pendant and/or endgroup of material G, NCOArR1(OSiR2R3)_(n)R1ArOCN, to form aromaticcarbamate pendant and/or end group of material G2,H₂NCOOArR1(OSiR2R3)_(n)R1ArOOCNH₂. When heated, the aromatic carbamatependant and/or end group of material G2 will breakdown, or decompose, toform aromatic amine pendant and/or end group of material G3,H₂NArR1(OSiR2R3)_(n)R1ArNH₂, and carbon dioxide, CO₂, gaseous byproduct.

In an embodiment of the present invention, the cure of material G isoptimized to react most of the aromatic cyanate ester pendant and/or endgroups. Thus, production of carbon dioxide in the interior of thepackage cavity, when moisture and heat are present, may be minimized.

In another embodiment of the present invention, instead of co-curingwith material G, the non-G materials may cure independently to form aninterpenetrating polymeric network (IPN) within the three-dimensionalnetwork of material K.

In an embodiment of the present invention, material K, as shown in FIG.8, has a novel composition that includes siloxane backbones that arelinked to triazine rings. The three-dimensional network of substitutedtriazine rings makes material K thermally stable. Thus, material K iscompatible with a 260-degree Centigrade reflow that may be part of asubsequent Lead-free solder process. Material K is also easilycollapsible so a very thin bond line thickness (BLT) may be achieved indie attach.

The material properties and mechanical properties of material K dependon the average distance between the triazine rings (cross-link density)in material K, which, in turn, is affected by the distance between thependant and/or end groups on the backbone and the branches, if present,in material A. Chain entanglement of material K depends on polarity andshape of the polymer and may occur if a critical chain length ormolecular weight is exceeded. In an embodiment of the present invention,a higher cross-link density will result in a a higher glass transitiontemperature, T_(g), a lower coefficient of thermal expansion (CTE), ahigher Young's modulus of elasticity, and a lower moisture absorption.

The glass transition temperature, T_(g), refers to the temperature atwhich a non-crystallizable or amorphous polymer changes properties as ittransitions between a glassy state (below the T_(g)) and a rubbery state(above the T_(g)). A polymer in the glassy state tends to be hard,stiff, brittle, and transparent while a polymer in the rubbery statetends to be soft, flexible, tough, and opaque.

The T_(g) of a homopolymer largely depends on the stiffness orflexibility of the repeating unit in the backbone (or main chain) of thepolymer and the functional (side) groups attached to the backbone. Thestiffness or flexibility depends on the ease of rotation about the bondsin the backbone and the side groups, as well as the size, bulkiness, andpolarity of the side groups. The T_(g) also depends on molecular weight,branching, and crosslinking of the polymer.

Block and graft copolymers usually have multiple T_(g) that are near theT_(g) for each constituent homopolymer. In contrast, alternating andrandom (or statistical) copolymers usually have a single T_(g) betweenthe T_(g) of the constituent homopolymers.

The CTE refers to the fractional change in dimension per unit change intemperature. Typically, the CTE of a material will increasesignificantly when the temperature is above the glass transitiontemperature, T_(g).

Young's modulus of elasticity, also known just as modulus of elasticity,refers to a ratio of stress to strain of a material that is placed intension, compression, or shear. Stress refers to a rate of applied loadto a cross-sectional area. Strain refers to a rate of change in length,or elongation, relative to an original length.

A good stress buffer located at the interface between two dissimilarmaterials will prevent warpage, bending, cracking, or delamination eventhough a large amount of stress, such as tensile stress, is induced bythermal cycling. In an embodiment of the present invention, themechanical properties of material K may be optimized to prevent damageto the materials on either side of the interface.

A low value of the modulus of elasticity will allow material K to serveas a good stress buffer between a silicon die and a substrate. A silicondie has a relatively low CTE of about 2-3 ppm/degree Centigrade.However, a ceramic (alumina) substrate has a CTE of about 6-9 ppm/degreeCentigrade, a plastic substrate has a CTE of about 20 ppm/degreeCentigrade, and an organic substrate may have a CTE of about 30ppm/degree Centigrade or greater.

The modulus of elasticity is related to the amount of stress that thedie attach adhesive will transfer to the silicon die when a mismatch inCTE exists between the die and the substrate. A die attach adhesive witha high modulus of elasticity will hold the die rigidly and transfer morestress to the die.

The mechanical properties of material K may be optimized to maintain adesired modulus of elasticity at a certain temperature above the glasstransition temperature, T_(g). The glass transition temperature may beabout −25 to 100 degrees Centigrade. The modulus of elasticity may beabout 100 MPa to 2 GigaPascals (GPa).

Material I is a filler that may be included, as needed, during thecuring of material G to form material K. A filler is a substance thatmay be blended with a polymer to improve material and mechanicalproperties, such as thermal conductivity and electrical conductivity. Inan embodiment of the present invention, a filler loading of about 50percent by weight or less is used. In another embodiment of the presentinvention, a filler loading of about 20-40 percent by weight may beused.

Material K may have a high CTE, such as about 50-200 ppm/degreeCentigrade, depending upon the cross-link density. The CTE is usuallylower for an aromatic compound and higher for an aliphatic compound. Inan embodiment of the present invention, material I may have a low CTE,such as about 2-3 ppm/degree Centigrade, so material I may be blendedwith material G to reduce the CTE of material K. For comparison, ametallic solder bump may have a CTE of about 20-30 ppm/degreeCentigrade.

In another embodiment of the present invention, material I may beblended with material G to increase fracture toughness of material K,such as to about 2 MPa/m². Fracture toughness of a material is relatedto the amount of energy that is required to propagate a crack in thematerial.

In still another embodiment of the present invention, material I may beblended with material G to improve workability or processability. In anembodiment of the present invention, material I may be a diluent. Adiluent is a low-molecular-weight material that may serve a functionsimilar to that of a solvent. In an embodiment of the present invention,curing may result in linking of a diluent, such as material I, with ahigher-molecular-weight material, such as material G, to reduceviscosity. Any residual diluent that remains unlinked may subsequentlyvaporize or outgas and may contaminate the interior of the packagecavity.

Examples of diluents may include liquid epoxies and liquid cyanateesters. Examples of liquid epoxies include Epon resin 234, Epon resin235, Epon resin 813, Epon resin 815C, Epon resin 862, Epon resin 8021,Epon resin 8101, Epon resin 8111, and Eponex resin 1510.

Material I may include particles of a material, such as silica (SiO₂).In an embodiment of the present invention, a silica loading of about 50percent by weight or less is used. In another embodiment of the presentinvention, a silica loading of about 20-40 percent by weight of silicamay be used.

In an embodiment of the present invention, the silica particles may havea nominal diameter of about 0.5-5.0 microns (um). In another embodimentof the present invention, the silica particles may have a nominaldiameter of about 5.0-10.0 um. In still another embodiment of thepresent invention, the silica particles may have a nominal diameter ofabout 10.0-25.0 um.

Bleed out is a separation of a monomer vehicle phase and the fillerduring staging or cure that results in a spread of the resin away from adeposit location, such as the die bond area. Resin flow may contaminate,or otherwise interfere, with the wire bond pads. Bleed out may bestrongly affected by temperature. In an embodiment of the presentinvention, the curing reaction, such as for cyanate ester, may beexothermic. The resultant elevation in temperature may reduce viscosityof the monomer, thus increasing bleed out.

Fumed silica may be used as a thixotrope to increase viscosity ofmaterial G. A higher viscosity may reduce or inhibit bleed out ofmaterial G. Fumed silica is finely divided silica that may be formed byreaction of silicon tetrachloride (SiCl₄) and hydrogen (H₂).

The thixotropy of a liquid refers to the flowability of the liquid. Athixotropic liquid has a great surface tension and may form a convexsurface. A higher thixotropic index at room temperature will allow amaterial to shear thin. Silica may be blended with material K to reduceBLT to about 0.5-4.0 mil.

Silica may also be blended with material G to reduce dielectric constantof material K. In an embodiment of the present invention, the dielectricconstant may be about 2.0-3.0.

Polydimethylsiloxane polymers may include highly ionic, cohesive mainchains (backbones) and nonionic, less cohesive side chains. In general,the polydimethylsiloxanes may have a helical structure, with thesiloxane bonds directed inward. Introducing siloxane structures into apolymer increases the space occupied by the polymer. Siloxane structuresalso provide elasticity and flexibility to a polymer because they tendnot to interact with each other.

In an embodiment of the present invention, the silicone backbone ofmaterial K has a sufficiently low modulus of elasticity. However, inanother embodiment of the present invention, material I may be blendedwith material G to increase flexibility of material K. Material I mayinclude elastomers, such as polytetrafluoroethylene (Teflon),cross-linked olefins, and acrylates.

Material J is a coupling agent that may be included, as needed, duringthe curing of material G to form material K. In an embodiment of thepresent invention, about 2 percent by weight or less of the couplingagent is used. In another embodiment of the present invention, about 1percent by weight or less of the coupling agent is used.

A coupling agent, such as material J, improves an interfacial bondbetween a polymer (continuous phase), such as material G, and a filler(discontinuous phase), such as material I. In an embodiment of thepresent invention, material J may modify a surface of material I tolower surface tension, enhance wetting, and improve adhesion.

In an embodiment of the present invention, a coupling agent, such asmaterial J, includes a first functional group that reacts with thepolymer, such as material G, and a second functional group that reactswith the surface of the filler. Examples of coupling agents includeorganosilanes, organotitanates, organozirconates, aluminates, silicateesters, metal acrylates, and methacrylates. Other examples of couplingagents include compounds that contain a copolymerizable group and achelating ligand, such as phosphine, mercaptan, and acetoacetate.

Die attach may be followed by wire bonding and encapsulation. Wire bondpads are located on the tier(s) or co-planar with the surface of thesubstrate. A wire-bonding machine welds fine wires, typically ofAluminum or Gold, between each die bond pad on the die and theappropriate wire bond pad on the substrate. Wire bonding may alsoprovide interconnections for die-to-die as well as for die-to-passivecomponent, such as capacitor, resistor, or inductor.

Finally, encapsulation protects the die and the bond wires frommechanical damage during subsequent handling and processing.Encapsulation may involve liquid dispensing or transfer molding.

Many embodiments and numerous details have been set forth above in orderto provide a thorough understanding of the present invention. Oneskilled in the art will appreciate that many of the features in oneembodiment are equally applicable to other embodiments. One skilled inthe art will also appreciate the ability to make various equivalentsubstitutions for those specific materials, processes, dimensions,concentrations, etc. described herein. It is to be understood that thedetailed description of the present invention should be taken asillustrative and not limiting, wherein the scope of the presentinvention should be determined by the claims that follow.

Thus, we have described a die attach adhesive and a method of formingsuch an adhesive.

1. A method comprising: providing a material A, said material Acomprising a siloxane backbone with a hydride functional group; reactingsaid material A with a material B in the presence of a catalyst to forma material C, said material B comprising an alkenyl functional group andan aromatic carbonate functional group; heating said material C to forma material D, said material D comprising a phenol functional group; andreacting said material D with a material E and a material F to form amaterial G, said material E comprising a cyanogen halide, said materialF comprising an acid acceptor, said material G comprising an aromaticcyanate ester functional group.
 2. The method of claim 1 wherein saidmaterial A comprises a linear siloxane backbone.
 3. The method of claim1 wherein said material A comprises a branched siloxane backbone.
 4. Themethod of claim 1 wherein said material A comprises a cyclic siloxanebackbone.
 5. The method of claim 1 wherein said material B comprises astyrene substituted with a tertiary-butyl carbonate functional group. 6.The method of claim 1 wherein said catalyst comprises a complex of ametal.
 7. The method of claim 1 wherein said material E comprisescyanogen bromide (BrCN).
 8. The method of claim 1 wherein said materialF comprises an alkyl amine.
 9. A method comprising: providing a materialG, said material G comprising a polyorganosiloxane that includesaromatic cyanate ester functional groups; and heating said material G inthe presence of a catalyst to form a material K.
 10. The method of claim9 wherein said catalyst comprises an organometallic compound.
 11. Themethod of claim 9 wherein said catalyst comprises a metal chelate. 12.The method of claim 9 wherein said catalyst is provided in a solution ofalkyl liquid phenol.
 13. The method of claim 9 wherein said material Kcomprises siloxane backbones that are linked to triazine rings.